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G—PHYSICS

G01—MEASURING; TESTING

G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR

G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable

G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infra-red, visible, or ultra-violet light

G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infra-red, visible, or ultra-violet light with attenuation or whole or partial obturation of beams of light

G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infra-red, visible, or ultra-violet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells

G01D5/36—Forming the light into pulses

G01D5/38—Forming the light into pulses by diffraction gratings

Abstract

A scale (1) and scale reader (16) are disclosed for measuring displacement in two
directions (X and Y). One embodiment of the scale shows a chequered pattern
forming a matrix of marks (12) for providing a light pattern to a diffraction grating
(2). The diffraction grating has a similar chequered pattern and provides an
interference light pattern to a photodetector array (4). The interference pattern
enables the detector to determine movement in each of the two directions.

Description

A known form of opto-electronic scale reading apparatus for measuring relative
displacement of two members comprises a scale on one of the members, having
scale marks forming a periodic pattern, a read head provided on the other member,
means for illuminating the scale, periodic diffraction means for interacting with light
from the scale marks to produce interference fringes having movement relative to the
read head and detecting means in the read head responsive to the fringes to produce a
measure of the displacement.

An example of such apparatus is disclosed in EP-A-0 207 121 and also
US-A-4,974,962, each of which shows the means for illuminating and the periodic
diffraction means in the read head. US-A-4,926,566 discloses a method of
producing a scale, in the form of a flexible tape produced by rolling, the pitch of the
scale marks being 20µm or 40pm for example. The illuminating means, the
diffraction means and the detecting means responsive to the fringes may be
integrated in the read head in the manner described in US-A-5,302,820.

However, in the above basically only displacement in one linear direction may be
measured. One way to measure displacement in two orthogonal linear directions is
to use two read heads in one body at right angles to one another. However,
measurement errors, particularly Abbé errors, can result from the necessary spacing
between the two read heads. Such an arrangement may also be expensive or not
compact.

Another known form of opto-electronic scale reading apparatus is disclosed in
US-A-5,204,524. The scale comprises a diffraction grating co-operating with at
least one other grating on a read head to produce interference fringes, some or all of
which move relative to the read head during a displacement of the read head relative
to the scale and the measurement is a count of such fringes. Such a scale has to be of
diffraction quality, and the accuracy and reliability of the measurement depends on
such parameters as the regularity of the spacing of the scale marks, the sharp
definition of the edges of the marks and the freedom of the scale from scratches and
like imperfections. Such a scale can be expensive to produce and protect especially
when the scale has to be relatively large.

It is also known to increase the number of signals obtainable from any two adjacent
marks of the scale by phase quadrature interpolation and known scale-reading
apparatus can be subject to phase errors.

In each of US-A-5,204,524 and US-A-5,576,537 there is shown apparatus for
measuring displacement in each of two directions between two members wherein use
is made of a scale capable of producing diffracted orders. Such a scale must
necessarily have marks which have a single periodicity in each of the two directions
and there is no indication of the presence of secondary periodicities defining
departures from a single periodicity in each of the two directions, or of a spatial filter
having a pass band defining a maximum such departure.

According to the present invention, there is provided measurement apparatus
comprising a scale extending in two directions and scale reader apparatus for
determining displacement during relative movement between the scale and the scale
reader apparatus in each of the two directions, the scale comprising a matrix pattern
having periodicity in the two directions, the scale reader apparatus comprising a
scale illuminator, a diffraction grating structure and a light detector, in use the
illuminator acting to illuminate the scale, the grating structure causing light from the
scale to be formed into an interference pattern having light of varying intensity and
the detector acting to detect the interference pattern and produce an indication of the
displacement in each of the two directions, the measurement apparatus being
characterised in that the grating structure provides a grating in one of the two
directions interlaced within a grating in the other of the two directions.

The present invention will now be described, with reference to the accompanying
drawings wherein:-

Figure 1 is a view of an embodiment of apparatus according to the invention;

Figure 2 is a representation of the scale of Figure 1;

Figure 3 is a diagram showing the optical layout and operation of the
apparatus;

Figure 4 is a representation of the interference pattern formed by part of the
apparatus of the first embodiment;

Figure 5 is a representation of the interference pattern formed by part of the
apparatus of a second embodiment according to the present invention;

Figure 6 is a diagram showing the response of a filter; and

Figure 7 is a diagram showing a band of scale periodicities.

In the following embodiments of this invention, the diffraction mechanism takes
place wholly within the read head. The scale is required merely to provide a pattern
of light sources. The scale is not required to be a diffraction grating and the marks
on the scale do not have to be of diffraction quality. The read head performs an
optical convolution, i.e. the interference pattern generated in the plane of the detector
array is a convolution of the scale with a pattern which is substantially sinusoidal in
the directions of measurement. This makes the read head substantially free from
phase quadrature errors.

The two directions in which the embodiments principally measure displacement are
hereinafter referred to as the "X" and "Y" directions, although orthogonality between
the X and Y directions is illustrated, the invention is not restricted so.

Referring to Figure 1, reference numeral 1 designates an X-Y scale for attachment to
a member whose displacement is to be measured in the X and Y directions. Scale 1
comprises a matrix of marks defining a pattern with periodicity in the X and Y
directions. Reference numeral 2 designates a two-dimensional diffraction grating
providing a grating in the X direction interlaced with a grating in the Y direction for
interacting with light reflected from the scale 1, the latter being illuminated by a
light-emissive diode (LED) source 3.

Inside a read head 16 (which by way of example also includes the grating 2 and the
LED 3), is a two-dimensional array of silicon photo-detectors. More particularly,
but by way of example only, array 4 comprises sets 5 and 6, each of strips of photo-detectors
aligned in the Y direction, and sets 7 and 8, each of strips of photodetectors
aligned in the X direction. The sets 5,6,7 and 8 are "L" shaped in Fig 1 but
other shapes are possible e.g. rectangular.

In use of the apparatus, by virtue of the two-dimensional matrix structure of scale 1
and the two-dimensional grating 2, in contrast to conventional linear encoders where
fringes are produced, spots of light at the array 4 are produced as a result of the
diffraction process. As scale 1 moves relative to the read head, movement of the
spots in the X direction is detected by the photo-detectors of the sets 5 and 6 to
produce measurement signals (13a) Ax, Bx, Cx, Dx . . . and the movement of the
spots in the Y direction is detected by the photo-detectors of the sets 7 and 8 to
produce measurement signals (13b) Ay, By, Cy, Dy ....

The scale 1 is typically a plate with periodic marks in the X and Y directions. The
marks may be defined by a single periodicity P1 in each of the X and Y directions,
or several periodicities, forming a band, the "scale band". The scale band includes
the dominant periodicity P1 among a range of secondary periodicities, which may be
produced by random variations in the periods of the marks. Figure 2 shows the
intensity profile 9 of such a scale 1, including such random variations represented as
randomly distributed reflective regions 10. The structure includes regions 12 with
period P1 in both the X and Y directions. This scale profile may be more
economical to produce than a scale with only a single periodicity in each direction.
The apparatus includes a filter consisting of the grating 2 and by a sampling region
11 spanning a portion of the area of the scale. P1 lies within the pass band of the
filter. The filter responds to the light pattern produced by the scale 1 and acts on the
detector array 4 to produce the signals 13a and 13b.

The regions 10 and 12 define the intensity distribution of the scale. Referring to
Figure 3, the grating 2 may be an amplitude grating, typically a Ronchi grating,
having periodicity in both the X and Y directions. Within the field of "Fourier
imaging", the phenomenon of "self-imaging" of periodic masks is used.

In the first embodiment, this phenomenon requires for these types of grating that the
following expressions are satisfied:
1/u+1/v=λ/(nxD22)D2/D3=u/(u+v)D2/D1 =v/(u+v)
wherein:

u =

the distance between a generating plane 14 and the grating 2;

v =

the distance between the grating 2 and the detector array 4;

λ =

the wavelength of the light;

D1 =

the pitch in both the X and Y directions of a plurality of said point
sources lying in the plane 14 and co-operating to form the interference
pattern;

D2 =

the pitch in both the X and Y directions of the grating 2;

D3 =

the pitch in both the X and Y directions of the elements 5, 6, 7 and 8 of
the detector array 4;

n =

a positive integer.

Note that P1, D1, D2 and D3 may have different values in the X and Y directions.
They should more correctly be referred to as P1X,P1Y, D1X, D1Y, D2X, D2Y, D3X
and D3Y. However, for simplicity of explanation only, it is assumed that:
P1 = P1X=P1YD1 =D1X=D1YD2=D2X=D2Y
and
D3 = D3X = D3Y

The plane 14 lies in the XY directions and contains a notional point source 15 of
substantially monochromatic light. This arrangement produces an interference
pattern (Figure 4) substantially similar in pattern to grating 2, this interference
pattern being a self-image of the grating.

The head 16 and the scale 1 are matched by making the pitch D1 of the read head
equal to the pitch P1 of the scale. The head is positioned relative to the scale so that
the scale 1 is substantially coincident with the generating plane 14. Notional light
sources 21 (Figure 4) are then the actual sources defined by the light reflected from
surface features of the scale.

During relative movement of the head 16 and the scale 1, the resulting movement of
the notional light sources 21 in the generating plane 14 in the direction X produces a
corresponding movement of the interference pattern (Figure 4) in the direction X,
relative to the read head 16 and the resulting movement of the notional light sources
in the direction Y produces a corresponding movement of the interference pattern in
the direction Y relative to the read head. If u and v are equal, the amount of
movement of the interference pattern relative to the read head is the same as that of
the relative movement of the head and the scale. A hypothetical line sensor 19
parallel to the Y direction and situated in the plane 22 of the interference pattern will
detect fluctuations in light intensity as the interference pattern passes across it with
some component of movement in the X direction and a hypothetical line sensor 20
parallel to the X direction and situated in the same plane will detect fluctuations in
light intensity as the interference pattern moves across it with some component of
movement in the Y direction. The detector array 4 has areas 5 and 6 with a pitch D3
in the X direction equal to the pitch of the periodicity of the interference pattern in
the X direction and areas 7 and 8 with a pitch D3 in the Y direction equal to the pitch
of the periodicity of the interference pattern in the Y direction. It is arranged that the
plane of the detector array 4 coincides with the plane 22 of the interference pattern.

In the second embodiment the parameters of the head are given by:
1/u+1/v=λ/[(n+½)×D22]D2 / D3 = 2u / (u + v)D2/D1=2v/(u+v)

The restriction of equation (1) does not apply at all in this case. However, equation
(4) should be applied when n is low and/or the light is sufficiently monochromatic.
Otherwise, the contrast of the interference pattern is substantially independent of
wavelength and broad-band light, e.g. white light, may be used. In this embodiment,
the pitch of the periodicity of the interference pattern (Figure 5) that is formed is
dependent only on the ratio u/v and not on the absolute values of u and v. There is
some loss of interference pattern contrast in this case, but this is overcome by using a
phase grating for the grating 2. Generally, this embodiment would be the preferred
embodiment of this invention.

The pitch D1 is also referred to as the "nominal periodicity" of the filter, and the
filter may be said to be tuned to read only those marks 12 of the scale 1 which have
the nominal periodicity of the filter or lie within the pass band of the filter.

A housing supports the grating 2 and the detector array 4 at the spacing v and a
support means supports the housing relative to the scale 1 at the distance u between
the scale 1 and the grating 2.

It can be shown on the basis of Fourier theory that an optical convolution is
performed between the scale pattern 10 and the interference pattern (Figure 5) due to
a single light source 15 illuminating the grating 2. Since sections through the
interference pattern parallel to the X direction and sections through the interference
pattern parallel to the Y direction are substantially sinusoidal, this convolution is a
spatial filtering of the light distribution of the scale in favour of the spatial
periodicity in the X and Y directions of the interference pattern. In other words, the
head 16 is a tuned spatial filter.

The filtering action is strengthened in this case by a second convolution between the
interference pattern (Figure 5) and the detector array 4.

The convolutional character of the read head 16 causes it to be substantially
independent of angular misalignment with respect to the scale 1, particularly about
the Z axis. The read head is thus substantially immune to quadrature phase error
caused by angular misalignment since the grating 2 and the detector array 4 are fixed
one relative to another and the interference pattern (Figure 5) has a fixed alignment
with the lines of the grating 2. Therefore, the head 16 can be set up, relative to the
scale, by simple mechanical methods, such as setting gauges and it is not normally
necessary, during setting up, to monitor the phase of the signals 13 and make
adjustments in the head position to eliminate phase errors as between the respective
signals 13.

The spatial filter is designed to pass some scale periodicites but not others. The
periodicities which are passed by the filter constitute two bands (hereinafter the filter
bands) FBx and FBy (Figure 6) of periodicities, one such band FBx being in the X
direction and the other band FBy being in the Y direction. The "X" filter band and
the "Y" filter band may be determined separately by at least one of the following:

the illuminated or sampling region 11 of the scale 1 (Figure 1);

the optical aperture of the grating 2;

the optical aperture of the detector array 4;

the structure of the grating 2;

the structure of the detector elements 5,6,7 and 8;

the wavelength and spectral bandwidth of the light;

the degree to which the scale 1 scatters the light as opposed to reflecting it
specularly;

the position, extent and divergence of the source 3.

Depending on the geometry of the filter, one of these constraints may dominate over
the others, or more than one may act together to set the filter bandwidths. The
constraint or constraints setting the filter bandwidth in the X direction need not
necessarily be the same as the constraint or constraints setting the filter bandwidth in
the Y direction. Any of these constraints may be used to set the size of the filter
band in each direction.

The region 11 may be illuminated over an area smaller in X or Y or both dimensions
than an area corresponding to the greatest possible aperture of the grating 2 in which
case the effective aperture is smaller than said greatest possible aperture. In practice,
given that the scale has the periodicity P1 in each of the X and Y directions, the filter
F is designed to match the periodicity P1. To cope with a given tolerance in the
actual periodicity of the scale, i.e. in the spacing of the marks 12, due to
manufacturing tolerances, the pass band of the filter is made sufficiently wide to
include that tolerance. However, the dominant scale periodicity P1 needs to be
detectably present on the scale in the sense of lying within said sampling region 11
in both X and Y directions and within the pass band of the filter in both X and Y
directions.

Figure 6 is a diagram showing the pass bands FBx and FBy of the filter. The surface
17 represents the whole response of the filter in terms of the contrast of the
interference pattern (figure 5) for different scale periodicities SPx and SPy. An
interference pattern contrast above a plane 18 is sufficient to produce signals 13a and
13b (Figure 1).

For simplicity of explanation only, it is now assumed that FBx = FBy = FB and
SPx = SPy = SP.

So long as the periodicity P 1 lies within the band FB in the X and Y directions, the
filter can respond to it and produce a signal of acceptable amplitude. While being
substantially uniform within the sampling region, P1 may vary over the extent of the
scale 1. So long as P1 varies in such a way that within the sampling region,
wherever this may be with respect to the scale, P1 always lies within the filter band
FB an acceptable signal 13 will be produced. The filter responds in sympathy with
any such changes in the periodicity.

This is acceptable for a given error tolerance. However, the arrangement has the
advantage of relatively good freedom from quadrature phase error. In a typical
example, the nominal periodicity in the X and Y directions is 20µm and the width of
the pass band is 0.1 µm in each direction for a sample region 11 of I Omm square. If
P1 varies over a range of 0.05 µm, the error tolerance would have to be 2.5mm per m.
However, as little phase quadrature error is introduced, this can be compensated for
and reduced, typically to 20µm per m.

Figure 7 shows a band of scale periodicities (PB 1 x, PB 1 y) present within the
sampling region 11 and including the dominant periodicity P1 within the band in
both the X and Y directions. If the dominant periodicity coincides with the nominal
periodicity of the filter, the filter response is in accordance with the nominal
periodicity. If as shown in Figure 7, the dominant periodicity P1 does not coincide
with the nominal filter periodicity Pf, then the filter response is in accordance with
the dominant periodicity P1. So long as P1 lies within the filter band, an acceptable
signal 13 is produced.

The scale 1 may be a flat plane or a curved surface. It may be continuous in one or
more directions, for example, as a band or cylinder.

A second diffraction grating may be used between the grating 2 and the detector
array 4 to match the pitch of the interference pattern to the pitch of the detector
array, for example by making use of Moire effects.

The grating 2 may be a phase grating or a Ronchi grating.

The grating 2 and/or the detector array 4 may be tilted by rotation about an axis in a
plane containing the X and Y directions in order to increase the tolerance of the read
head to misalignment with respect to the scale, particularly to increase the tolerance
on distance u.

The patterns of marks illustrated for the scale and grating are chequered but other
regular patterns would give satisfactory results e.g. rows of circles. It will be noted
that the orientation of the squares of the chequered pattern is diagonal to the X and Y
directions shown. Thus movement in X or Y by the readhead results in an overall
dark then not dark photodetection during the "diagonal" movement. However the
same effect would not happen if the X and Y directions were aligned with the sides
of the squares of the chequered pattern.

Claims (6)

Measurement apparatus comprising a scale (1) extending in two directions
(X,Y) and scale reader apparatus for determining displacement during relative
movement between the scale and the scale reader apparatus in each of the two
directions, the scale comprising a matrix pattern (12) having periodicity in the two
directions, the scale reader apparatus comprising a scale illuminator (3), a diffraction
grating structure (2) and a light detector (4), in use the illuminator acting to
illuminate the scale, the grating structure causing light from the scale to be formed
into an interference pattern having light of varying intensity and the detector acting
to detect the interference pattern and produce an indication of the displacement in
each of the two directions, the measurement apparatus being characterised in that the
grating structure provides a grating in one of the two directions interlaced within a
grating in the other of the two directions.

Measurement apparatus as claimed in claim 1 wherein the interlaced gratings
of the grating structure are formed as a matrix pattern having periodicity in the two
directions.

Measurement apparatus as claimed in claim 2 wherein the matrix patterns of
the scale and grating are each formed as a chequered pattern.

Measurement apparatus as claimed in any one of claims 1,2 or 3 wherein the
scale reader apparatus is in the form of a readhead (16) and the scale illuminator,
grating structure and light detector are within the readhead.

Measurement apparatus as claimed in any one of claims 1 to 4 wherein the
light detector is an array of photodetectors arranged in a plurality of sets (5,6,7,8)
each set having a plurality of strips of photodetectors being aligned in one of the two
directions.

Measurement apparatus as claimed in claim 5 wherein the sets are "L"
shaped and are fitted together.